The present invention relates generally to photolithography masks, and more particularly to methods to fabricate photolithography masks. Still more particularly, the present invention relates to methods to repair defects in material substrates used to fabricate these masks.
Lithography is a process by which the pattern of a lithography mask is transferred to a layer of resist deposited on the surface of the semiconductor wafer. Photolithography is a variation of lithography techniques whereby ultra-violet (UV) radiation is utilized to expose a resist layer.
The lithography mask used in photolithography is generally known as a photomask, which is used to expose resist in selected areas. Typically, the photolithography mask consists of chrome areas supported by a high-quality quartz plate. The former is opaque to UV radiation, while the latter is transparent to UV radiation. By selectively exposing areas of oxide deposits on a wafer, and after a number of substquent steps including but not limiting to etching and doping, semiconductor integrated circuit (ICs) devices may be developed.
As IC devices become more complex and compact, precision in photolithography is increasingly required. One demand is to reduce the minimum-feature size of these devices, which in turn requires a proportional reduction in mask minimum-feature size. However, as the mask minimum-feature size becomes smaller, defect size also becomes smaller. Unfortunately, even a small, undetected defect may cause fatal defects in the final photolithographic image printed on the wafer-substrate. Since the occurrence rate of most defects is roughly inversely proportional to defect size, as mask minimum-feature size becomes smaller, the number of fatal defects increases in mask production.
In photomasks produced for the “phase-shift photolithography” technology, defects may cause even more problems because of the required additional processing associated with etching of steps into the quartz substrates. Therefore, in addition to surface pits originating from voids in the original quartz substrate material, surface pit defects can be easily produced during the quartz-etching steps of phase-shift mask fabrication.
In most cases photomasks are required to be perfect. It is well known in the industry that as a result of defects, yields of masks for advanced devices can be so low that the concern goes beyond price. Indeed, many, if not most advanced photomasks require reworking to repair defects before they are deemed acceptable.
Several pit defect repair techniques have been developed, and some of them are illustrated in FIGS. 1-3. In FIG. 1, a photomask 100 has a quartz pit defect 102 that is filled with deposited material 104, which has a refraction index that is close to the refraction index of quartz. A probe 106 is used to remove excess material. FIG. 2 shows another technique in which the original pit 102 is deepened to a two-pi depth 108. FIG. 3 illustrates still another technique, cutting one or more steps 110 to surround the pit.
However, all these techniques are slow and expensive, primarily due to the time required for removing a certain amount of substrate material. What is needed is an improved method for repairing mask quartz defects.